There is considerable demand for short-wavelength laser sources such as green, blue and UV lasers. One known approach to create such a light source is to utilize red or infra-red laser diodes, which are widely available in a variety of configurations. These diodes, in combination with nonlinear elements made of optically nonlinear materials, can produce short-wavelength frequency-doubled radiation by means of nonlinear frequency conversion such as second harmonic generation (SHG) in a nonlinear optical element.
In frequency converting laser devices such as frequency doubling devices a beam of light of a narrow wavelength range, commonly referred to as a fundamental beam having a fundamental frequency, illuminates a nonlinear crystal which doubles the output frequency of the light. The input light beam can be typically in an infrared region and the output light will be in the visible portion of the spectrum. For example, if a 980 nm laser wavelength light is passed through a nonlinear crystal such as LiTaO3 blue light with wavelength of 490 nm will be output.
For efficient frequency conversion, the fundamental frequency beam and the frequency-converted beam have to propagate through nonlinear material with the same phase velocity, a requirement commonly referred to as a phase matching condition. Refractive index of most materials though is wavelength dependent, which makes phase matching of the fundamental and frequency-converted beams difficult. Various methods are used to satisfy the phase matching condition. These methods include passing light through a nonlinear crystal at a specific angle or passing the light through a so called periodically poled (PP) nonlinear crystal, i.e. a crystal with periodically arranged ferroelectric domains of inverted polarity. In such cases the specific angle used for passing light through the crystal or the period of the inverted polarity zones determines a specific wavelength range of the incident light that can be phase matched.
In general, the phase matching wavelength has to be very precise, and small deviations from the optimum value can cause significant loss of frequency conversion efficiency. Typically, the wavelength of the laser has to stay within ˜0.1 nm to 0.02 nm from the optimum phase matching wavelength. This requirement puts a stringent condition on input laser sources for frequency doubled light, typically requiring the use of frequency-selective elements such as gratings to narrow and stabilize the frequency range of laser radiation.
A number of prior art solutions for frequency conversion and frequency doubling of laser diode emission utilizing frequency selective elements have been disclosed. For example, U.S. Pat. No. 5,185,752, in the names of Welch et al., describes arrangements for efficiently coupling light between a laser diode and a second-harmonic generator which feature external resonant cavities that include a feedback grating fabricated on the second-harmonic generator. U.S. Pat. No. 5,644,584, in the names of Nam, et al. describes a tunable blue laser diode having a distributed Bragg reflector (DBR) or distributed feedback (DFB) tunable diode laser coupled to a quasi-phase-matched waveguide of optically nonlinear material. In U.S. Pat. No. RE35,215, Waarts et al. describe a semiconductor laser light source which employs a Littman grating coupled to a back facet, providing short wavelength light by means of frequency doubling of red or infrared light from a high power flared resonator type laser diode, or a MOPA (master oscillator power amplifier) type laser diode.
These inventions provide solutions wherein power and frequency stabilization requirements are met through the use of complex laser structures or complex nonlinear element arrangements. An alternative approach is to use high power semi-conductor lasers of simple cavity design, such as edge emitting 980 nm laser diodes commonly used to pump erbium-doped fiber amplifiers, in an extended cavity arrangement with frequency stabilization provided by an external frequency selective reflector such as a fiber Bragg grating (FBG). These commercially available inexpensive single spatial mode semiconductor chips have an antireflection coated front facet, and can generate over 1 watt of power in continuous operation, provided that optical feedback from the FBG into the laser diode is optimized, typically at a feedback level when about 3% of the laser radiation is returned back into the laser diode. U.S. Pat. No. 5,544,271 to Lim describes a nonlinear optical generator of such a type which includes a semiconductor laser diode with an anti-reflection coated facet, an optical fiber with a fiber Bragg grating incorporated in it and a nonlinear material for nonlinear frequency conversion.
Although the aforementioned inventions appear to perform their intended functions, they provide solutions wherein the laser sources operate in a single-frequency regime, with lasing on a single longitudinal mode of the laser cavity. Single frequency single mode lasing however is known to exhibit spectral and power instabilities and mode hops due to aging, changes in temperature, pump current etc. Contrary to single frequency operation, a multi-frequency laser having several longitudinal modes present in its optical spectrum is known to provide more stable output and to allow avoiding mode hops and other instabilities associated with changing lasing conditions.
Several prior art solutions that use multiple longitudinal mode multifrequency operation of laser diodes with external fiber Bragg grating reflectors for spectral and/or power stabilization of their output have been proposed. U.S. Pat. No. 5,485,481 to Ventrudo et al. describes an FBG-stabilized multifrequency laser wherein an optical feedback provided by the FBG is small compared to a feedback provided by reflections from an output facet of the laser diode, resulting in a so-called coherence collapse regime of laser operation, when the laser diode is forced into a state of chaotic but stable wide-band multifrequency lasing. High-power FBG-stabilized lasers of this type based on 980 nm laser diodes operating in a multifrequency coherence collapse regime are commercially available. However, the coherence collapse regime of operation together with a substantially broadband reflection spectrum of the FBG used in 980 nm pump lasers results in a broad laser linewidth of the order of 0.3–1 nm, far exceeding typical linewidth requirements of ˜0.02–0.1 nm or less for efficient SHG in periodically-poled nonlinear materials. Therefore, although the multifrequency operation of such high-power FBG-stabilized lasers does provide more stable output, this solution may be less efficient for the purpose of nonlinear frequency conversion.
U.S. Pat. No. 5,724,377 to Huang describes a method of reducing power fluctuations for an FBG-stabilized laser diode caused by mode hops due to variations in laser current by using an FBG which reflection bandwidth is 2 to 4 times bigger than a longitudinal mode spacing of the laser diode. This method, however, does not necessarily ensure multi-frequency operation of a laser diode and hence does not provide spectral stabilization required for stable frequency conversion.
Suppression of spectral, as well as power, instabilities of a laser output provided by multifrequency lasing can be of especial importance when this output is used for nonlinear frequency conversion. Power and spectral instabilities such as mode hops can be transformed with amplification into power instabilities of the frequency converted light due to a quadratic dependence of the converted light power on the optical power of the input fundamental radiation and due to the aforementioned strong spectral dependence of the frequency conversion efficiency on the fundamental wavelength in periodically poled nonlinear elements.
The use of multifrequency operation has another important advantage for nonlinear frequency conversion which has been generally overlooked in the prior-art solutions for sources of frequency converted radiation. Namely, using a source of fundamental laser radiation having multiple longitudinal modes in it spectrum allows achieving higher nonlinear conversion efficiency. Indeed, the conversion efficiency has been found to depend on the number of longitudinal modes N which are present in the optical spectrum of the laser radiation, as follows: Eff(N)=Eff(1)*k(N), where a factor k(N)=(2−1/N), and Eff(1) is a single-mode conversion efficiency. More details are given, for example, by Risk et al. in Section 2.2.6 of a book entitled Compact Blue Green Lasers, Cambridge University Press, 2003, and by Zernike and Midwinter, in Applied Nonlinear Optics, John Wiley and Sons, 1973. For a large number of modes the efficiency of nonlinear frequency conversion is therefore twice as high as for a single mode case, and approximately 50% higher in a two-mode case, which translates into up to two times higher power of frequency converted light for the same input power of the fundamental radiation.
An object of this invention is to provide a multifrequency external cavity laser diode arrangement for generation of short-wavelength radiation through nonlinear frequency conversion such as frequency doubling, which is capable of achieving stable high power operation using simple, low cost optics and assembly.